remove ruby 2.1 and 2.2 which have reached EOL
remove ruby 2.1 and 2.2 which have reached EOL

see these links for context:
- https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/
- https://www.ruby-lang.org/en/news/2018/06/20/support-of-ruby-2-2-has-ended/

flavorjones/concourse-gem